Short-Term Borrowings (Schedule of Short-term Debt) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Demand Notes	$ 3,229	$ 3,094
Short-term Bank Loans and Notes Payable	7	167
Federal Home Loan Bank, Advances, Short-term	3,500	3,800
Securities Sold under Agreements to Repurchase	482	0
Other Short-term Borrowings	400	400
Short-term borrowings	7,618	7,461
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,229	3,094
Short-term Bank Loans and Notes Payable	7	167
Federal Home Loan Bank, Advances, Short-term	0	0
Securities Sold under Agreements to Repurchase	0	0
Other Short-term Borrowings	0	0
Short-term borrowings	3,236	3,261
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Short-term Bank Loans and Notes Payable	0	0
Federal Home Loan Bank, Advances, Short-term	3,500	3,800
Securities Sold under Agreements to Repurchase	482	0
Other Short-term Borrowings	400	400
Short-term borrowings	$ 4,382	$ 4,200